HER2 (ErbB2), a receptor tyrosine kinase oncogene, promotes cell growth and transformation of cancer cells. Strong expression of HER2 in breast cancer has been associated with poor prognosis. Also, decreased expression of the p27 protein was shown to correlate with cancer development and poor survival rates. Studies have shown that HER2 overexpression correlates with p27 downregulation. While link between HER2 activation and low p27 expression in cancers is established, our understanding of how precisely HER2 downstream signal pathways regulate p27 degradation, what these pathways do, and why their regulations are so important, remains elusive. The goal of this proposal is to elucidate the molecular mechanism of mammalian constitutive photomorphogenesis 9 signalosome (COP9) subunit 6 (CSN6)-mediated p27 degradation and to illustrate it as a signal target of HER2/Akt signals involved in degrading p27. The COP9 signalosome was originally identified from plant (Arabidopsis) mutants that mimic light-induced seedling development when grown in the dark. Mammalian cells have COP9 signalosome too, but the biological functions are largely unknown. We propose three Specific aims: (1) To determine the role of HER2-Akt signaling in regulating CSN6 and tumorigenicity. (2) To determine CSN6's role in regulating p27 function. (3) To determine the contribution of CSN6 in development and ErbB2-mediated mammary tumorigenesis. To further understand the molecular mechanism by which HER2- Akt-CSN6 pathway regulates p27 degradation, we will investigate the molecular regulations of CSN6 with Akt, a HER2 downstream kinase, and define the functions of CSN6 as a regulator of p27 degradation. We have generated mice carrying targeted disruption of the CSN6 gene with a plan to study CSN6-mediated p27 regulation in vivo. However, CSN6 knockout mice are embryonic lethal. We will generate CSN6 conditional knockout mice to determine the influence of CSN6 on the p27 degradation and on the development of mammary gland. Finally, we will use the MMTV-ErbB2 transgenic mouse as a model system to study the role of CSN6 in ErbB2 regulated cancer. Given the pivotal role of ErbB2-regulated p27 degradation in tumorigenesis, an understanding of how the function of p27 is regulated will provide important insight about the compound layers of p27 regulation and will help in the development of therapeutic interventions for cancer in which p27 is deregulated.